Mass spectrometry

ABSTRACT

In a double-beam mass spectrometer, the two spectral outputs are given a small relative time displacement and superimposed without any peaks overlapping. Only a single data processing channel is required with only a single analogue-to-digital converter for computer processing of the results.

United States Patent 11 1 Merren 1 Nov. 6, 1973 [54] MASS SPECTROMETRYFOREIGN PATENTS OR APPLICATIONS 1 lnventorl Thomas Merren, Cheshire,1,161,432 8/1969 Great Britain 250 419 ME England [73] Assignee:Associated Electrical Industries Primary Examzner-James W. LawrenceLnmted London England Assistant Examiner-C. E. Church [22] Filed: May28, 197i Att0rney--Watts, Hoffmann, Fisher & Heinke [21] Appl. No.:148,114

[30] Foreign Application Priority Data v [57] ABSTRACT June 17, 1970Great Britain 29,499/70 In a double beam mass Spectrometer the twospectral outputs are given a small relative time displacement 2% 250/4134 36 and superimposed without any peaks overlapping. Only 250 D a singledata processing channel is required with only 1 0 care a singleanalogue-to-digital converter for computer References Cited processingof the results.

UNITED STATES PATENTS 8 Claims, 4 Drawing Figures 3,242,333 3/1966Yamamoto 250/419 D Ion Accelerating Voltage Supply putar I Data DlgltolCombmahon-i- Pagoessmg AZD' Com PATENTEDRUV BIQYS 3,770,337

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DIQIt'EX Computer Signal Delay Means 0m Processing A/D Channe! SignalCombination Cir'cmr PATENTEUNHY ems 3770.337 SHEET l1, 0F 4 IonAmelerafing VoH'bge Supply DeFIecHon Voltage ProporrioningMms SignalCombinahon Digifal i l Prooessin Circuif (,hamneA Computer PrinY-Ouh 1MASS SPECTROMETRY CROSS-REFERENCE To RELATED APPLICATIONS Ser. No.'73,072; filed Sept. 17, 1970 byBrian Noel Green. Multiple Slit Assemblyfor Mass Spectrometers Ser. No. 119,250; filed Feb. 26, 1971 by BrianNoel Green and Robert Alexander McDowell and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to mass spectrometers; and more particularly to double beam massspectrometers of the kind described in the above reference Powers andGreen applications.

2. Description of the Prior Art In the analysis of substances with amass spectrometer, ions of a substance being analysed are generated inan ion source and then expelled from the ion source as an ion beam. Thebeam passes through amagnetic analyser in the case a single-focusingmass spectrometer; and through an electrostatic analyser and then amagnetic analyser in the case of a double-focusing mass spectrometer. I

During a given analysis, the ion accelerating voltage or the magneticfield in the magnetic analyser may be varied to cause a scanning effectwhich causes ions of different mass-to charge ratiosto be collected bythe ion collector of the mass spectrometer. The collection of ions ofdifferent mass-to-charge'ratios during a scan causesthe spectrometer toproduce an output which is a mass spectrum of the substance beinganalysed. I

.In the above referenced Powers and Green applications, there aredisclosures of, amongst other things, the use of two ion sources whichsimultaneously and independently produce two ion beams which can be ofdifferent substances. The two ion beams are passed through a commonanalyser system and separately collected by two independent ioncollectors to produce two mass spectra. The use of acommon analyserensures that ions of the same mass-to-charge ratio in each of the beamsare given identical deflections. Thereby the two mass spectra maybemutually correlated to a very high degree of accuracy.

- Amongst many other advantages, a double beam mass spectrometerprovidesthe very important capability of chemical mass marking. Inchemical mass marking, one of the ionbeams is formed from a referencesubstance, such as perfluorokerosene, which provides a spectrumcontaining peaks at several known mass-to-charge ratios. Since thereference spectrum is accurately correlated with the spectrum of theunknown substance, the reference peaks act as accurate markers forming acalibrated scale from which the mass-to-charge ratios of peaks of theunknown substance can be interpolated. (In non-chemical mass marking,the relatively difficult and inaccurate process of analytical fieldmeasurement is necessary).

There is an ever increasing tendency to employ compouter processing ofmass spectrometric analyses,

2 with the attendant advantages of speed, accuracy, and the eliminationof tedious manual calculation and interpretation of results. The dualoutputs of a double beam mass spectrometer may be recorded for laterprocessing by a computer, or utilised in real time. Owing to thesimultaneous occurrence of the two outputs, they would require separatetreatment in prior art arrangements. Whenconverting the analogue outputsto digital signals compatible with the computer, two analogue-to--digital converters would be required. This would double the cost andoperating difficulties of a significant part of the system.

SUMMARY OF THE INVENTION 1 The present invention provides a method andapparatus for combining signals representative of the simultaneousspectral analysis of two substances thereby permitting single channelprocessing of the combined signal.

In accordance with one aspect of the present invention, a double beammass spectrometer is provided forsimulta'ne'ously enabling themassspectral'analysis of two substances such as an unknown and a referencesubstanceThe two mass spectra produced by the analysis are given a smallrelative time displacement so that they can be combined without anypeaks overlapping. The resulting single composite signal has no-loss ofspectral data compared to the two signals before combination, and thecomposite signal requires only a sin.- gle analogue-to-digital converterfor conversion to a digital form suitable for real-time processing in adigital computenp The-relative displacement can be accomplished in a 7number of ways. A preferred method is to displace one of the twoconventional collector slits of the mass spectrometer in the plane ofdeflection of the ion beam. Thus ions of the same mass-to-charge ratioin the different beams are collected at slightly different times duringa scan. Alternatively the electrical signals representing one of thespectra can be given a time delay, or both spectra could'be delayed bydifferent amounts. As a further alternative, the two ion beams could bepassed through different electrostatic fields in'the electrostaticanalyser. These different electrostatic fields can be achieved bysplitting one or both of the sectors forming the electrostatic analyserin a plane midway between the planes of thetwo ion beams, and applyingslightly different voltage to the part sectors.

Accordingly, it is the principal object of the present invention toprovide novel and improved methods and apparatus for combining into acomposite signal the spectral analyses of a plurality of substances.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspectivediagram of a priorart double beam mass spectrometer;

FIG. 2 is a schematic perspective diagram of a preferred embodiment ofthe invention;

FIG. 3 is a schematic perspective diagram of another embodiment of theinvention; and

FIG. 4 is a schematic perspective diagram of a further embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is shown inhighly schematic form the general layout of an M.S. 30 double-beam massspectrometer as sold by AEI Scientific Apparatus Inc. The M.S.3O massspectrometer is a double-focusing mass spectrometer, and in common withsingle-beam double-focusing mass spectrometers, possesses anelectrostatic analyser l and a magnetic analyser 12. Two ion sources 14and 16 each produce a respective ion beam 18 and 20 (representeddiagrammatically in the drawing) which arethen directed through theanalysers l0 and 12 for deflection and mass separation in known manner.One of the ion beams contains ions of a reference substance producingpeaks of accurately known mass numbers. After leaving the magneticanalyser 12, the ion beams 18 and 20 each pass through a respectivecollector slit 22 and 24 in a collector slit assembly 26 thereafter toimpinge on respective ion collectors 28 and 30. The outputs of the ioncollectors 28 and 30 are amplified in known manner by electronmultipliers and applied to a paper chart printer 32 of known type.

When the magnetic field in the magnetic analyser 12 is scanned, theoutput of the printer 32 is a chart bearing two traces, each of which isa mass spectrum of the substance from which ions are formed'in therespective one of the ion sources Hand 16. Owing to the indentity of theanalysing fields to which the ion beams 14 and 16 are subjected, theelectric signals supplied by the ion collectors are such that at anytime throughout the scan, both signals represent the same mass number ineach spectrum. The advantage of the spectral correlation is mitigated bythe necessity for providing dual data processing channels, each with itsown analogueto-digital converter, as a spectrometer/computer interfaceif on-line computer processing of results is required, in a manner oftenemployed with single-beam mass spectrometers. In FIGS. 2, 3 and 4 andthefollowing description, applicant shows how this disadvantage have beenomitted both for clarity and since they are not part of the inventionnor are they require for an understanding of the invention).

Referring now to FIG. 2, the basic M.S. 30 mass spectrometer isemployed, but with a modified collector slit assembly 226 wherein twocollector slits 222 and 224 have relatively different positions in therespective planes of deflection of the ion beams 18 and 20. Thepositional difference D between the slits 222 and 224 has the effectthat during a scan, ions of any given mass-to-charge ratio in one of theions beams reaches its respective ion collector at a slightly difierenttime that that at which ions of the same mass-to-chargeratio in theother ion beam reach the other ion collector. A suitable time differenceis between 1.5 and milliseconds for a ten second scan, depending on theresolutions of the two beams. (An ideally suitable slit assembly isdisclosed in the above referenced Green and Mc- Dowell application).

The outputs of the ion collectors 28 and 30 are combined in a suitablesignal combination circuit 34 which performs scalar addition of the twosignals to produce a single composite signal which resembles the outputof a single-beam mass spectrometer when analysing a substance producingfrequent peaks. This single signal is then processed in a D830 dataprocessing unit 36, as sold by AEI Scientific Apparatus Inc., to producean output indicative of the substances being analysed. Only a singledata processing channel 38 is required, the channel 38 needing only oneanalogue to digital converter 40.

Thus, entirely contrary to what might have been expected to be required,the invention enables singlebeam data processing equipment to beemployed with a double-beam mass spectrometer. The invention alsoenables mass measurement to be carried at low resolution. This is afeature which is unique to plural beam systems.

The invention is based on the fact that when dealing with samples havingpeaks with mass numbers not exceeding about 300, the actual massdisplayed by a peak produced by ions of a fraction of nominal massnumber N (where N is, of course, an integer) seldom lies outside therange N 0.1 and N 0.35. In these circumstances it is possible todisplace the reference spectrum so that its peaks lie in the free zonesbetween actual masses N 0.35 and N 0.9, thus keeping the reference peaksclear of the sample peaks and thereby indentifiable without confusion. I

Collector slit displacement is not the only way of achieving the objectsof the invention.

FIG. 3 shows an alternative embodiment wherein a standard M830 massspectrometer is again employed. The electrical output of one of the ioncollectors, (30) is time delayed in a suitable signal delay means 42,and then combined with the undelayed output of the other ion collector.(28) in the signal combination circuit 34, the resulting signal beingpassed to the D830 data processing unit (36) as before. The signal delaymeans 42 may take any known form, for example an electrical delay line,or (particularly for longer delays and/or slower scans) an ultrasonicdelay line. A known form of continuous loop tape recorder withrelatively displaced record and playback heads could be utilised as thedelay means 42. For a 10 second scan, a delay of between 1.5 and 10milliseconds is suitable, depending on the resolutions with which thetwo beams are collected.

FIG. 4 shows a third alternative embodiment of the invention forachieving spectral displacement. The M830 mass spectrometer is providedwith a dual electrostatic analyser 410 wherein the normally unitaryinner electrostatic sector is spit into two sectors 412 and 4l4.whichtogether with the normal unitary outer sector 416 effectively forms twoside-by-side electrostatic analysers. The standard electrostaticanalyser (10) is normally supplied from the ion accelerating voltagesupply via a potentiometric chain of resistors (not shown), and for thepurposes of the FIG. 4 embodiment, the ion accelerating voltage from asupply 7 418 is passed through a delfection voltage proportioning means420 which delivers correctly proportioned deflection voltages to thesectors 412, 414 and 416. The correct proportioning of the deflectionvoltages ensures that as in the FIG. 2 embodiment, ions of a givenmass-to-charge ratio in one beam reach the respective ion collector at aslightly different time during a scan than ions of the samemass-to-charge ratio in the other beam reach the other ion collector.The resulting signals arecombined and processed as before.

The proportioning means 420 may take the form of a relatively standardpotentiometric chain modified by the provision of a further tap,slightly displaced from the tap for supplying the normal inner sector.As an alternative to splitting the inner sector, the outer sector may besplit; or both inner and outer sectors may be split.

Other modifications and variations may be made to the invention.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

1 claim:

1. A double-beam mass spectrometric analytical system comprisingvariable analytical field producing means to produce a variableanalytical field; ion source means to produce two ion beams and directthem through the analytical field; two ion collectors each disposed tocollect one of the ion beams; spectral signal producing means to producea spectral signal from each of the ion collectors in dependence upon themass-to-charge ratios of collected ions; spectral signal timedisplacementmeans to produce a relative time displacement between thoseparts in each of thespectral signals corresponding to the collection ofions of the same mass-to-charge ratios by the respective collectors,said spectraltime displacement means comprising a collector slitassembly defining two collector slits, one in the path of each beam, theslits being rela tively displaced such that ions of the samemass-tocharge ratio in each beam pass through their respective collectorslit at different times during a scan; displaced spectral signalcombination means to combine the relatively displaced spectral signalsto form a combined spectral signal wherein peaks from one spectrum liein the inter-peak spaces of the other spectrum; a single data processingchannel including a single analogue-todigital converter to process andconvert the combined spectral signal into a digital computer compatiblesignal; and a digital computer adapted to receive said compatiblesignal, said computer being adapted and programmed to act upon datareceived to interpolate the mass numbers of peaks of one spectrum fromthe mass numbers of peaks of the other spectrum.

2, A double-beam mass spectrometric analytical system comprisingvariable analytical field producing means to produce avariable-analytical field; ion source means to produce two ion beams anddirect them through the analytical field; two ion collectors eachdisposed to collect one of the ion beams; spectral signal producingmeans to produce a spectral signal from each of the ion collectors independence upon the mass-to-charge ratios of collected ions; spectralsignal time displacement means to produce a relative time displacementbetween those parts in each of the spectral signals corresponding to thecollection of ions of the same mass-to-charge ratios by the respectivecollectors, said spectral displacement means comprising differential iondeflection field producing means to produce differential deflection ofions of the same mass-to-charge ratio in the different beams such thatduringa scan, ions of the mass-to-charge ratio in the different beamspass through the mass spectrometer at different times; displacedspectral signal combination means tocombine the relatively displacedspectral signals to form a combined spectral signal wherein peaks fromone spectrum lie in the inter-peak spaces of the other spectrum; asingle data processing channel including a single analogue-to-digitalconverter to pro cess and convert the combined spectral signal into adigital computer compatible signal; and a digital computer adapted toreceive said compatible signal, said computer being adapted andprogrammed to act upon data received to interpolate the mass numbers ofpeaks of one spectrum from the mass numbers of peaks of the otherspectrum.

3. A method of operating a double beam mass spectrometer comprising ionsource means to produce two ion beams, a common ion beam analyserproducing an ion analysing field, an ion collector slit assemblyincluding a pair of ion collector slits which are relatively displaced,and two ion collectors producing ion impingement dependent signals,comprising the steps of:

a. forming one of the ion beams from a substance to be analysed;

b. forming the other ion beam from a reference substance; I

c. passing both beams through the analyser while scanning the ionanalysing field so that the beams impinge upon the collectors to producesignals indicativeof the presence in the beams of ions of mass-to-chargeratios dependent on the instantaneous value of the analysing field andoverthe duration of the scan producing two mass spectra eachrespectively indicative of the composition of the substances from whichthe ion beams were formed, each spectrum comprising one or more peakseach indicative of a particular specific mass; giving the two spectra arelative time displacement by passing-the two ions beams through thepair of relatively displaced collector slits before they impingeupon'the collectors, such that peaks in one spectrum occur duringinter-peak spaces in the other spectrum; and,

e. combining the relatively time displaced spectra to form a singlecomposite spectrum. 4. A method of operating a double beam massspectrometer comprising ion source means to produce two ion beams, acommon ion beam analyser producing an ion analysing field, anelectrostatic analyser, and two ion collectors producing ion impingementdependent signals, comprising the steps of:

a. forming one of the ion beams from a substance to be analysed; b.forming the other ion beam stance; c. passing both .beams through theanalyser while scanning the ion analysing field so that the beamsimpinge upon the collectors to produce signals indicative of thepresence in the beams of ions of mass-to-charge ratios dependent on theinstantaneous value of the analysing field and over the duration of thescan producing two mass spectra each respectively indicative of thecomposition of the substances from which the ion beams were formed, eachspectrum comprising one or more peaks each indicative of a particularspecific mass;

from a reference subd. giving the two spectra a relative timedisplacement by subjecting the ion beams to different electrostaticfields within the electrostatic analyser, such that peaks in onespectrum occur during inter-peak spaces in the other spectrum: and

e. combining the relatively time displaced spectra to form a singlecomposite spectrum.

5. A method of analysing a first substance comprising the steps of:

a. ionising the first substance and forming a first ion beam therefrom;

b. ionising a second substance and forming a second ion beam therefrom;

c. passing the first and second ion beams through a common analyticalfield wherein ions in each of the beams are deflected according to themass-tocharge ratios of the ions;

d. passing the first and second ion beams through an ion collector slitassembly including a pair of ion collector slits which are relativelydisplaced;

e. collecting at any instant ions of a single mass-tocharge ratio ineach of the beams;

f. varying the analytical field to vary the mass-tocharge ratios ofcollected ions and thereby form two relatively time displaced massspectra one of each of the substances being analysed, each spectrumcomprising one or more peaks, each peak representing the presence in therespective ion beam of ions of a mass-to-charge ratio proportional tothe instantaneous analytical field at the respective time of collectionof the ions, the peaks in the spectrum of the first substanceconstituting a first series of peaks, the peaks in the spectrum of thesecond substance constituting a second series of peaks the relative timedisplacement between the two spectra being such that the first series ofpeaks occur during inter-peak spaces in the second series of peaks;

g. combining the relatively time displaced spectra to form a singlecomposite spectrum; and

h. interpolating the mass numbers of the first series of peaks from thesecond series of peaks to provide an analysis of the constitution of thefirst substance.

6. The method of claim wherein said second substance is a referencesubstance and said second series of peaks are reference peaks occurringat known mass numbers.

7. A method of analysing a first substance comprising the steps of:

a. ionising the first substance and forming a first ion beam therefrom;b. ionising a second substance and forming a second ion beam therefrom;

c. passing the first and second ion beams through an electrostaticanalyser, and subjecting the ion beams to different electrostatic fieldswithin the electrostatic analyser;

d. passing the first and second ion beams through a common analyticalfield wherein ions in each of the beams are deflected according to themass-tocharge ratios of the ions;

e. collecting at any instant ions of a single mass-tocharge ratio ineach of the beams;

f. varying the analytical field to vary the mass-tocharge ratios ofcollected ions and thereby form two relatively time displaced massspectra, one of each of the substances being analysed, each spectrumcomprising one or more peaks, each peak representing the presence in therespective ion beam of ions of a mass-to-charge ratio proportional tothe instantaneous analytical field at the respective time of collectionof the ions, the peaks in the spectrum of the first substanceconstituting a first series of peaks, the peaks in the spectrum of thesecond substance constituting a second series of peaks, the relativetime displacement between the two spectra being such that the firstseries of peaks occur during inter-peak spaces in the second series ofpeaks;

g. combining the relatively time displaces spectra to form a singlecomposite spectrum; and,

h. interpolating the mass numbers of the first series of peaks from thesecond series of peaks to provide an analysis of the constitution .ofthe first substance.

8. The method of claim 7 wherein said second substance is a referencesubstance and said second series of peaks are reference peaks occurringat known mass numbers.

1. A double-beam mass spectrometric analytical system comprisingvariable analytical field producing means to produce a variableanalytical field; ion source means to produce two ion beams and directthem through the analytical field; two ion collectors each disposed tocollect one of the ion beams; spectral signal producing means to producea spectral signal from each of the ion collectors in dependence upon themass-to-charge ratios of collected ions; spectral signal timedisplacement means to produce a relative time displacement between thoseparts in each of the spectral signals corresponding to the collection ofions of the same mass-to-charge ratios by the respective collectors,said spectral time displacement means comprising a collector slitassembly defining two collector slits, one in the path of each beam, theslits being relatively displaced such that ions of the samemass-to-charge ratio in each beam pass thrOugh their respectivecollector slit at different times during a scan; displaced spectralsignal combination means to combine the relatively displaced spectralsignals to form a combined spectral signal wherein peaks from onespectrum lie in the inter-peak spaces of the other spectrum; a singledata processing channel including a single analogue-to-digital converterto process and convert the combined spectral signal into a digitalcomputer compatible signal; and a digital computer adapted to receivesaid compatible signal, said computer being adapted and programmed toact upon data received to interpolate the mass numbers of peaks of onespectrum from the mass numbers of peaks of the other spectrum.
 2. Adouble-beam mass spectrometric analytical system comprising variableanalytical field producing means to produce a variable analytical field;ion source means to produce two ion beams and direct them through theanalytical field; two ion collectors each disposed to collect one of theion beams; spectral signal producing means to produce a spectral signalfrom each of the ion collectors in dependence upon the mass-to-chargeratios of collected ions; spectral signal time displacement means toproduce a relative time displacement between those parts in each of thespectral signals corresponding to the collection of ions of the samemass-to-charge ratios by the respective collectors, said spectraldisplacement means comprising differential ion deflection fieldproducing means to produce differential deflection of ions of the samemass-to-charge ratio in the different beams such that during a scan,ions of the mass-to-charge ratio in the different beams pass through themass spectrometer at different times; displaced spectral signalcombination means to combine the relatively displaced spectral signalsto form a combined spectral signal wherein peaks from one spectrum liein the inter-peak spaces of the other spectrum; a single data processingchannel including a single analogue-to-digital converter to process andconvert the combined spectral signal into a digital computer compatiblesignal; and a digital computer adapted to receive said compatiblesignal, said computer being adapted and programmed to act upon datareceived to interpolate the mass numbers of peaks of one spectrum fromthe mass numbers of peaks of the other spectrum.
 3. A method ofoperating a double beam mass spectrometer comprising ion source means toproduce two ion beams, a common ion beam analyser producing an ionanalysing field, an ion collector slit assembly including a pair of ioncollector slits which are relatively displaced, and two ion collectorsproducing ion impingement dependent signals, comprising the steps of: a.forming one of the ion beams from a substance to be analysed; b. formingthe other ion beam from a reference substance; c. passing both beamsthrough the analyser while scanning the ion analysing field so that thebeams impinge upon the collectors to produce signals indicative of thepresence in the beams of ions of mass-to-charge ratios dependent on theinstantaneous value of the analysing field and over the duration of thescan producing two mass spectra each respectively indicative of thecomposition of the substances from which the ion beams were formed, eachspectrum comprising one or more peaks each indicative of a particularspecific mass; d. giving the two spectra a relative time displacement bypassing the two ions beams through the pair of relatively displacedcollector slits before they impinge upon the collectors, such that peaksin one spectrum occur during inter-peak spaces in the other spectrum;and, e. combining the relatively time displaced spectra to form a singlecomposite spectrum.
 4. A method of operating a double beam massspectrometer comprising ion source means to produce two ion beams, acommon ion beam analyser producing an ion analysing field, anelectrostatic analyser, and two ion collectors producing ion impingementdependent signAls, comprising the steps of: a. forming one of the ionbeams from a substance to be analysed; b. forming the other ion beamfrom a reference substance; c. passing both beams through the analyserwhile scanning the ion analysing field so that the beams impinge uponthe collectors to produce signals indicative of the presence in thebeams of ions of mass-to-charge ratios dependent on the instantaneousvalue of the analysing field and over the duration of the scan producingtwo mass spectra each respectively indicative of the composition of thesubstances from which the ion beams were formed, each spectrumcomprising one or more peaks each indicative of a particular specificmass; d. giving the two spectra a relative time displacement bysubjecting the ion beams to different electro-static fields within theelectrostatic analyser, such that peaks in one spectrum occur duringinter-peak spaces in the other spectrum: and e. combining the relativelytime displaced spectra to form a single composite spectrum.
 5. A methodof analysing a first substance comprising the steps of: a. ionising thefirst substance and forming a first ion beam therefrom; b. ionising asecond substance and forming a second ion beam therefrom; c. passing thefirst and second ion beams through a common analytical field whereinions in each of the beams are deflected according to the mass-to-chargeratios of the ions; d. passing the first and second ion beams through anion collector slit assembly including a pair of ion collector slitswhich are relatively displaced; e. collecting at any instant ions of asingle mass-to-charge ratio in each of the beams; f. varying theanalytical field to vary the mass-to-charge ratios of collected ions andthereby form two relatively time displaced mass spectra one of each ofthe substances being analysed, each spectrum comprising one or morepeaks, each peak representing the presence in the respective ion beam ofions of a mass-to-charge ratio proportional to the instantaneousanalytical field at the respective time of collection of the ions, thepeaks in the spectrum of the first substance constituting a first seriesof peaks, the peaks in the spectrum of the second substance constitutinga second series of peaks the relative time displacement between the twospectra being such that the first series of peaks occur duringinter-peak spaces in the second series of peaks; g. combining therelatively time displaced spectra to form a single composite spectrum;and h. interpolating the mass numbers of the first series of peaks fromthe second series of peaks to provide an analysis of the constitution ofthe first substance.
 6. The method of claim 5 wherein said secondsubstance is a reference substance and said second series of peaks arereference peaks occurring at known mass numbers.
 7. A method ofanalysing a first substance comprising the steps of: a. ionising thefirst substance and forming a first ion beam therefrom; b. ionising asecond substance and forming a second ion beam therefrom; c. passing thefirst and second ion beams through an electrostatic analyser, andsubjecting the ion beams to different electrostatic fields within theelectrostatic analyser; d. passing the first and second ion beamsthrough a common analytical field wherein ions in each of the beams aredeflected according to the mass-to-charge ratios of the ions; e.collecting at any instant ions of a single mass-to-charge ratio in eachof the beams; f. varying the analytical field to vary the mass-to-chargeratios of collected ions and thereby form two relatively time displacedmass spectra, one of each of the substances being analysed, eachspectrum comprising one or more peaks, each peak representing thepresence in the respective ion beam of ions of a mass-to-charge ratioproportional to the instantaneous analytical field at the respectivetime of collection of the ions, the peaks in the spectrum Of the firstsubstance constituting a first series of peaks, the peaks in thespectrum of the second substance constituting a second series of peaks,the relative time displacement between the two spectra being such thatthe first series of peaks occur during inter-peak spaces in the secondseries of peaks; g. combining the relatively time displaces spectra toform a single composite spectrum; and, h. interpolating the mass numbersof the first series of peaks from the second series of peaks to providean analysis of the constitution of the first substance.
 8. The method ofclaim 7 wherein said second substance is a reference substance and saidsecond series of peaks are reference peaks occurring at known massnumbers.